The present invention relates to an acceleration sensor module which is installed in a vehicle body to detect the change of acceleration occurring in case of collision or accident of the vehicle.
Many vehicles are recently equipped with airbag devices which are installed in the instrument panels of vehicles for protecting drivers and passengers from being injured when the vehicles are involved in collisions or accidents. The components of these airbag devices are roughly classified into a sensing section, an adjusting section, a collision judging section, and an actuating section. The sensing section, positioned at a front region or a side region of the vehicle, detects a negative acceleration generating in response to the collision or accident of the vehicle. The adjusting section executes necessary adjustments (e.g. AD conversion) based the detection result. The collision judging section compares the adjusted detection result with a reference value to judge the occurrence of any collision or accident. Then, the actuating section actuates (i.e. inflates) the airbag.
FIG. 10 shows a conventional acceleration sensor module for an airbag device (refer to the Japanese Patent Application Laid-open No. 2001-334908) which includes an acceleration sensor 100, an AD converter 102, a communication control circuit 104, a data conversion circuit 106, and a driver/receiver 108.
The acceleration sensor 100 detects an acceleration of the vehicle measurable at a position where the acceleration sensor 100 is disposed. The AD converter 102 converts an acceleration value detected by the acceleration sensor 100 into a digital value (i.e. digital acceleration data). The AD converter 102 sends the digital acceleration data to the communication control circuit 104. The communication control circuit 104, operating in response to a request of ECU (electronic control unit), transmits the digital acceleration data via the data conversion circuit 106 and the driver/receiver 108 to a power source signal line 110.
According to the above-described conventional system, the acceleration sensor 100 detecting the change of acceleration is not always accurate. More specifically, the AD converter 102 converts the detected acceleration value into a digital signal in synchronism with the clock generated from an oscillator. The acceleration sensor 100 is selectable from various types. For example, the acceleration sensor 100 is a capacitive sensor which includes a stationary electrode and a movable electrode which are opposed to each other to arrange a capacitor between them. The movable electrode causes a displacement relative to the stationary electrode. For example, the gap between the movable electrode and the stationary electrode changes in response to a change of acceleration occurring due to collision or accident of the vehicle. The electrostatic capacity of the capacitor changes in accordance with the change of the gap between the electrodes.
The clock produced from the oscillator is also used in the communication section and/or in the collision judging section. The power source section is thus subjected to a large variation or fluctuation of the load appearing at both rise and fall timings of the clock. Accordingly, the output voltage of the power source section causes undesirable variation or fluctuation correspondingly (refer to FIG. 4).
In the process of converting the change of capacitance into a voltage value in the acceleration sensor 100, the power source voltage may include a variation or fluctuation (i.e. noise) in the case of using the clock generated from the oscillator. The accuracy of the capacitance-voltage conversion will deteriorate. As the power source voltage serves as the reference voltages for the AD conversion and the capacitance-voltage conversion, the converted values will include adverse effects of the variation occurring in the reference voltages.